Phosphites and phosphonites have been well regarded for years as efficient hydroperoxide decomposers, often giving synergistic effects in combination with the more efficient peroxy radical scavenging phenolic antioxidants. However some of the commonly used phosphites are hydrolyrically unstable, can contribute to corrosion and handling problems during transfer and mixing, and occasionally form "black specks" during extrusion at higher temperatures and longer residence times. Further, some show an adverse effect on organoleptic properties, which can limit their use in materials for packaging applications. Despite much continued work toward new phosphites and phosphonites with incrementally better properties, there has been a need for a phosph(on)ite-free phenolic antioxidant system for plastic materials.
Aside from its inherent safety (it is edible & GRAS) and its suitable physical properties, .alpha.-tocopherol, also known as vitamin E, is an effective scavenger of oxy radicals, such as hydroperoxyl, alkoxyl, hydroxyl, phenoxyl, etc. known. It is also a very reactive towards less electrophilic radicals such as alkyl, towards hydroperoxides, excited states of ketones, singlet oxygen, ozone, superoxide, nitrogen oxides, and other reactive species associated with oxidative damage. In many cases the relative reactivity of tocopherol has been found to be orders of magnitude faster than those of other phenolics. Its transformation products include materials which are also effective oxy radical and carbon radical scavengers.
Like most phenolics, .alpha.-tocopherol forms some colored transformation products. Some earlier attempts at its use for polyolefin stabilization have failed because investigators did not appreciate the opportunity to use the much lower amounts of tocopherol which its unusual reactivity affords. In many cases appropriately lower levels of tocopherol now give very good stabilization with acceptable color results, notably in polyethylenes. To improve results in more demanding applications, a formulation of .alpha.-tocopherol with glycerol and glycerol surfactants was developed.
Secondary antioxidants such as phosphites and phosphonites have played a very important role in the stabilization of polyolefins for years. As peroxide decomposers, they must compete effectively with the polymer substrate RH for the chain-propagating alkyl peroxy radicals (ROO.cndot.) and they must also form an efficient chain-terminating agent in this reaction with the ROO.cndot. radicals. The antioxidative efficiency of the phosphites are greatly affected by the polar and steric effects of the groups bound to phosphorus, decreasing with increasing electron-acceptor ability, and by the bulkiness of the substituent groups. They also must be hydrolytically stable to be blended and mixed thoroughly and homogeneously with the primary phenolic antioxidants. The hydrolytic stability of phosphites improves as the number of phosphorus-carbon bonds increase. Unfortunately, the most effective phosphites in the market today are not very hydrolytically stable and can cause mixing/handling problems to the polymer producers, as well as convertors and compounders. The commonly used phosphites occasionally form `black specks` during extrusion at high temperature and for long residence time. Besides hydrolytic instability problems, phosphites also show an adverse effect on the organoleptic properties, which can limit their use in the packaging industry. Therefore, there is a strong desire and interest to develop a phosph(on)ite free phenolic antioxidant system. It is also an object to provide a polyolefin composition that is stabilized with a phosph(on)ite free antioxidant system.